either you need to request/seek this specifically or they may have terms which they publish which states the basis on which content can be used. Even if their content is from MS or other big names you still need to seek permission or at least check their T&Cs

Well, how would anybody know who stole who's picture first.
I would say it's fine. As long as the website design is unique.

If its unique, then whoever designed that holds the copyright. In truth you only have to worry if you think the owner will protect or enforce their copyright. Try using a Coca Cola or MacDonalds image if you want to see what that feels like :)

I've found that if you email them and explain, they are often ok about it...they might just want some very discreet acknowledgemen. Its all down to how you feel about it and how likely they are to find out and do something about it - that usually is only a request to remove the images etc